The field of this invention is drive circuits for latch-type relays having separate set and reset coils.
Electromagnetic relays are well known devices which are commonly employed in industrial environments to switch relatively high voltages. Energizing the relay coil with the appropriate voltage causes the relay contacts to open or close, depending on the type of relay, to create either an open or closed circuit with a source of high voltage and a load, such as a motor or lamp. With most relays, the relay coil must remain energized in order to keep the relay contacts in the desired conduction state. If the excitation is removed from the relay coil, then the relay contacts change conduction states.
In some industrial applications, it is necessary to employ a large number of electromagnetic relays for controlling various lamps and motors or the like. Employing a large number of conventional relays requires a very substantial power supply because of the necessity to keep each relay energized to maintain its contacts in the desired conduction state. In such applications, latch-type relays having separate set and reset coils are often substituted for the conventional single coil relay. The latch-type relay avoids the need for constant energization because once the latch relay is set by momentarily energizing the set coil, the relay remains latched. Only when the reset coil is energized will the conduction state of the relay contacts change. Since only a momentary pulse of current is required to latch the contacts of the latch-relay, a much smaller power supply can be employed to drive a large number of relays.
The disadvantage of such latch-type relays is that the relay contacts can remain set after the power supply voltage has failed in contrast to the contact conduction state of a conventional relay which changes once the relay coil is no longer energized from the power supply.